1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a facsimile machine or a copying machine and, more particularly, to a multiple image forming apparatus in which an image is formed on a transfer member carried by a transfer member carrier.
2. Description of the Related Art
Conventionally, image forming apparatuses have been used in which a toner image formed on a photosensitive drum is transferred to a transfer member carried and transported by a transfer drum to complete image formation. FIG. 14 shows an example of such conventional image forming apparatuses.
The process of the recording operation of the image forming apparatus shown in FIG. 14 will be described in detail. When the apparatus starts operating, one of recording paper sheets is fed from a paper feeder unit and is conveyed to register rollers 102 by pre-conveyance rollers. At this time, the register rollers 102 are stopped. The conveyance operation of the pre-conveyance rollers is continued for a short time after the moment at which the leading end of the recording paper sheet is brought into abutment against the register rollers 102. When the recording paper sheet forms a predetermined loop (hereinafter referred to as "pre-register loop"), the pre-conveyance rollers are stopped.
Thereafter, the register rollers 102 and the pre-conveyance rollers are rotated in synchronization with an image recording start signal to feed the recording paper sheet onto a transfer drum 101. This operation is performed to enable an image to be formed at a predetermined position on the recording paper sheet.
On the other hand, a latent image is formed on a photosensitive drum 100 by performing image exposure with laser light E on the basis of an image signal. This latent image is developed with a toner.
The recording paper sheet is conveyed to an image transfer position T while being supported on the transfer drum 101, and the toner image on the photosensitive drum 100 is recorded by being transferred onto the surface of the recording paper sheet. The recording paper sheet is supported on the transfer drum 101 by one of several possible methods, i.e., an electrostatic attraction method, an air suction method and the like.
If in the thus-arranged apparatus a difference occurs between the speeds of the photosensitive drum 100 and the transfer drum 101, a deterioration is caused in the quality of the resulting image, that is, the transferred image is expanded or contracted or is blurred by rubbing. Ordinarily, to avoid this problem, the two drums are driven by the same drive source and, more preferably, the two drums are linked and the driving force is directly transmitted to them so that the speed difference therebetween is minimized.
If there is a difference between the recording paper conveying speed of the register rollers 102 and the peripheral speed of the transfer drum 101, a problem described below arises. If the recording paper conveying speed of the register rollers 102 is lower than the peripheral speed of the transfer drum 101, the recording paper sheet pulls the transfer drum 101 to change the speed of this drum after being supported thereon. It is also possible that, in the worst case, the recording paper sheet peels off the transfer drum 101.
Conversely, if the recording paper conveying speed of the register rollers 102 is higher than the peripheral speed of the transfer drum 101, the recording paper sheet starts forming a loop between the register roller 102 and the transfer drum 101 to cause and increase a force of pressing the transfer drum 101. When this force exceeds a certain magnitude, the speed of the transfer drum 101 is changed. This change causes a change in the speed of the photosensitive drum 100 linked to the transfer drum 101, resulting in expansion or contraction of the latent image formed during sheet conveyance.
To cope with this problem, a method has generally been practiced in which the recording paper conveying speed of the register rollers 102 is set slightly higher than the peripheral speed of the transfer drum 101 at the stage of designing so that there is no possibility of the recording paper conveying speed of the register roller 102 being lower than the peripheral speed of the transfer drum 101 with respect to any dispersion in certain part accuracy ranges. This is because only a small relative reduction in the recording paper conveying speed of the register rollers results in a considerable deterioration in image quality while a relative increase in the recording paper conveying speed in certain allowance is negligible.
A mechanism for setting the recording paper sheet free by separating a pair of register rollers 102 from each other has also been provided. This is intended to separate the register rollers 102 before a loop of the recording paper sheet between the register rollers 102 and the transfer drum 101 (hereinafter referred to as "post-register loop") formed by earlier registration pushes the transfer drum 101 (this separating operation hereinafter referred to as "register release") in order to increase the looping space so that the force of the recording paper sheet in the looping state is reduced.
However, the recording paper conveying speeds of the pre-register rollers and the register roller 102 are ordinarily set to the same speed because the pre-register rollers and the register rollers 102 simultaneously convey the same sheet for a certain period of time. Accordingly, the recording paper conveying speed of the pre-conveyance rollers is slightly higher than the peripheral speed of the transfer drum 101 and, therefore, a loop is also formed and increased between the pre-conveyance rollers and the transfer drum 101. This loop ceases to exist instantaneously when the trailing end of the recording paper sheet passes the pre-conveyance rollers. Then, the force acting on the transfer drum 101 changes abruptly to cause a change in the speed of the transfer drum 101. The shock of such a change may also be transmitted to the photosensitive drum 100 to affect the latent image during the formation on the photosensitive drum 100.
By considering this problem, a method may be used in which the pre-register or post-register looping space is maximized so that the curvature of the loop is reduced if the amount of loop is constant. However, it is difficult to provide such a large looping space and to limit the increase in the overall size of the apparatus. On the other hand, there is a limitation upon simply reducing the amount of loop since a certain effect of use of the loop must be achieved.
Recently, apparatuses having more complicated sheet feeder units have generally been developed. Such apparatuses require an elongated and complicated sheet path from the feeder unit to the register rollers. Under such circumstances, variation in sheet conveyance speed tends to increase and a certain amount of loop is indispensable also for absorbing variation in sheet conveyance speed.
A mechanism for removing the recording paper conveying speed may be provided for the pre-register rollers as well as for the register rollers to prevent occurrence of vibration when the trailing end of the recording paper sheet passes the pre-register rollers, as disclosed in EP 0 480 454 A2. Use of such a mechanism, however, necessitates an increase in manufacturing cost and is also disadvantageous in terms of space factor.